SWR Meter (7): Test results of log amp detector

 I ran tests on the response of the SWR detector PCB.  Everything appears to function well. The response graph is shown below

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It looks like I've got the noise problems worked out.  The response is very close to what is given in the data sheet.  There is a jog in the response at about -50 dBm.  This jog is there because I had to switch to an attenuator to get signals down below -50 dBm.  The attenuator changes the load on the signal generator slightly, resulting in the slight offset in response below -50 dBm.

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I am designing the circuit to measure power up to 300W.  The amplitude of the Vr port on the coupler is: 

$V_R=\frac{\sqrt{PR}}{N}$

where: 

• P is the power on the load
• R is the load resistance (50 ohm)
• N is the number of turns on the current transformer of the coupler. (20)

This gives an RMS voltage of 6.1 volts for VR for 300W on the load. 

The AD8307 datasheet define dBm as:

$dBm =10\log_{10}\left( \frac{V_{RMS}^2}{R}\cdot 1000 \right)  $

Using this formula,  6.1 volts is 28.7 dBm.   In order to keep the dBm input to the AD8307 to under 10 dBm,  I will add a 20 dB attenuator between the coupler and the AD8307.